Numerous processes exist for the removal of sulfur dioxide from flue gases, such as flue gases produced in electric power plants where a carbonaceous material, such as coal or oil, is combusted to produce heat and power. Such flue gases containing sulfur dioxide also contain fly ash from coal combustion, carbon and oils which sometimes pass through particulate collecting devices and are passed to the flue gas desulfurization unit and subsequently to aqueous waste slurries discharged from the flue gas desulfurization unit. When calcium scrubbing components such as limestone or lime are used to capture the sulfur dioxide, calcium-sulfur salts, such as solid calcium sulfite or calcium sulfate are formed and discharged in an aqueous waste slurry.
It is, of course, desirable to sell or use the calcium sulfite or calcium sulfate in such aqueous waste slurries so as to reduce costs, with such use being the production of gypsum or .alpha.-hemihydrate gypsum as a salable product. A problem exists, however, in that even minor amounts of grits, fly ash, carbon or oils that contaminate the gypsum product from flue gas desulfurization waste slurries seriously affect the quality of the gypsum product, since carbon and oil contaminants can cause release of paper from a core of a gypsum wallboard material, and carbon, oil or fly ash contaminants will discolor plaster formed from such a gypsum product. An additional problem exists in that when lime or limestone are used as the sulfur dioxide-capturing calcium components in a scrubber, poorly ground limestone grits or lime grits, coarse particles, may also be found in the aqueous waste slurry which are also not wanted in a wallboard or plaster byproduct. Coarse particles, if not removed, can cause lumps in the final gypsum material and are also detrimental to the color of the by-products.
Combustion burners that are operated to produce low volumes of nitrogen oxides (NO.sub.x burners) tend to produce more carbon than conventional burners, with excess carbon, at times, passing through an electrostatic precipitator. This can result in carbon reporting to the wet scrubber used for removing sulfur dioxide.
Known commercial solutions to the removal of such contaminants in aqueous waste slurries from flue gas desulfurization systems generally involve the use of a series of hydroclones. While such hydroclones are useful in removing coarse fractions from fine fractions, such solutions are limited and not totally effective.
It is an object of the present invention to provide a method for purifying an aqueous waste slurry from a flue gas desulfurization system, containing calcium sulfite or calcium sulfate solids, grit material, fly ash and carbon and oil, so as to allow use of the calcium sulfite or calcium sulfate in production of .alpha.-hemihydrate gypsum, plaster or plaster related products.
It is another object of the present invention to provide a method for purifying an aqueous waste slurry from a flue gas desulfurization system, containing calcium sulfite, grit material, fly ash, and carbon and oils, so as to provide an enhanced starting material for use in the production of .alpha.-hemihydrate gypsum or dihydrate gypsum.